This invention relates generally to the building of three-dimensional objects, and, more particularly, concerns an improved process for building three-dimensional objects based on electrophotograhic printing, wherein deformations found in completed three-dimensional objects caused by spillover effects of the building process are minimized.
The production of three-dimensional objects in an additive manner without the need for any tooling by numerous methods is well known. In such systems, a three-dimensional object is built layer-by-layer with each layer representing selected cross-sections thereof. Successive, adjacent layers representing corresponding successive, adjacent cross-sections of the object being built are formed and bonded together to produced the object involved.
One such system is described in U.S. Pat. No. 4,575,330. The system generates a three- dimensional object layer-by-layer in a process called stereolithography wherein a beam of UV laser is guided across the surface of a liquid UV-curable photopolymer according to a selected cross-section of that object. Areas exposed to the UV laser are polymerized and solidified to form a layer of solid plastic at or just below the surface. The completed layer is then lowered into the liquid UV-curable photopolymer and covered with a new layer of said liquid polymer and the laser shining step repeated for the generation of the next layer and, so on, until all the layers that make up the desired object is obtained.
Another alternative approach is described in U.S. Pat. No. 5,204,055. Powdered base material, such as a powdered ceramic or a powdered plastic is deposited in sequential layers one on top of the other. Liquid binder material is then selectively supplied to the layer of powdered base material using an ink-jet printing technique to bond the powdered base material together in accordance with a computer model of the three-dimensional object being formed. When all the layers composing the object are formed, unbounded powder is removed, resulting in the formation of the desired three-dimensional object.
The examples described hitherto, as well as many other systems that are in commercial use, are essentially point-to-point processes whereby three-dimensional objects are formed bit by bit, therefore the speed at which layers are formed is slower than if the layers are formed layer-by-layer. In view of this problem, there is considerable effort directed to designing systems which form three-dimensional objects layer-by-layer instead of bit by bit. U.S. Pat. No. 6,066,285, for example, discloses a method of solid freeform fabrication of a three-dimensional object using layer-by-layer deposition of at least one type of powder by electrophotographic printing. In an exemplary embodiment, a latent electrostatic image corresponding to a selected cross-section of the three-dimensional object to be formed is generated on an imaging member such as a photoconductor by applying light as a graphic pattern thereon. Thereafter, the latent electrostatic image is developed by depositing a part compositional powder and a support powder onto the surface of the imaging member whereon the part compositional powder adheres to the image portions of the latent image and the support powder adheres to the non-image portions of the latent image. The developed image is then transferred from the imaging member to a substrate. Subsequent layers are also formed in the same manner and deposited directly onto the previously deposited layer. The process is repeated until the three-dimensional object is fully formed.
A method is described in U.S. Pat. No. 6,376,148 which operates in substantially the same manner as described herein with reference to U.S. Pat. No. 6,066,285. In one preferred embodiment, a first layer of primary body-building powder material is deposited on a support platform and an electrophotographic printing means is operated to create a transferable binder powder image in accordance with the three-dimensional object. The transferable binder image, containing heat fusible materials, is then transferred to the layer of body-building material, melted, and allowed to permeate therethrough. Thereafter, the molten binder is hardened by cooling it to a temperature below its melting point. As the molten binder material solidifies, it bonds body-building material around it together to form a first cross-section of the three-dimensional object. Areas free of the binder material remain unbonded and stay as support material for subsequent layers. After that, a second layer of body-building powder material is deposited on the first layer and the step of generating and transferring a binder powder image is repeated to form a second layer of the three-dimensional object. Subsequent layers are formed in the same manner to produce the three-dimensional object.
The method and apparatus disclosed in U.S. Pat. No. 6,376,148 has several advantages. For one, the method builds three-dimensional objects more quickly than prior art systems as each layer of the object is built wholly whereas most prior art systems build objects in a point-by-point fashion. Also, the method has the advantage of being able to use a board array of materials, including both organic and inorganic substances as well as their mixtures, as the primary body-building powder. In contrast, some prior art systems like the one described hereinbefore with reference to U.S. Pat. No. 4,575,330, can use only one type of build material—photo-curable liquid resins. Further, numerous prior art electrophotographic printing based systems, such as the one described herein with reference to U.S. Pat. No. 6,066,285, shape each body-building layer via electrostatic attraction of the body-building powder material to the imaging member in an image-wise manner responsive to the electrostatic latent image thereon. The amount of material that can be deposited on the imaging member is dependent on the strength of the attractive force exerted by the imaging member on the body-building powder material which is dependent on the charge potential of the surface of the imaging member. Therefore, should adjustment to the thickness of the body-building material layer be desired, the imaging member must be charged to a different level than usual. However, electrical discharge from the imaging member, in the form of sparks, may occur if the voltage potential of said imaging member is increased to the point of air breakdown or ionization of air. Accordingly, the amount of body-building material that can be deposited in each layer forming step is limited. By contrast, the method and apparatus disclosed in U.S. Pat. No. 6,376,148 does not have this shortcoming as it performs the layer forming step in two separate steps, wherein the first is the deposition of the body-building powder material by conventional powder dispensing means and the second is the deposition of the binder material by electrophotographic printing. As such, thicker layers can be produced since a big portion of the materials needed in each layer is deposited in the first step, with the remaining coming from the second.
Another advantage associated with this method and apparatus is that the step of generating support structures for objects with features that are not self-supporting, like undercuts or overhangs, is unnecessary, as unbonded body-building powder material adjacent those that are bonded to form the three-dimensional objects, is retained in their original position throughout the build process and forms a natural support system for the objects that are being built. Lastly, the invention provides an apparatus that, through the use of mature technologies, including that of electrophotographic printing, is simple in design, efficient, and economical.
However, problems with the invention of U.S. Pat. No. 6,376,148 do exist. In order to bond loose body-building powder material to form three-dimensional objects, heat is applied to a binder material containing heat fusible materials by heat sources positioned near the object building zone to melt the binder, and cause it to permeate the body-building powder layer. The step of applying the heat must be carefully modulated so that the heat applied to one layer of powder is not transmitted to other layers beneath, otherwise the binder material that had solidified in preceding layers may re-melt and flow in a random manner into surrounding regions, including layers below. Consequently, layers adversely affected by the heat will lose their definition and result in deformed objects.
Another disadvantage associated with the invention of U.S. Pat. No. 6,376,148 is that it does not provide a process or apparatus which allows conductive materials, including all types of metals, to be used as the binder. Thus, the selection of binder material is limited to non-conductive ones, such as polymeric materials which, relative to metals and ceramics, have lower strength. Also, the service temperatures of polymeric materials are limited to a few hundred degree Celsius beyond which, softening of thermoplastic polymers or degradation of thermosetting polymers will occur. Accordingly, the mechanical strength, heat resistance and other properties of said objects are dependent on those of the binder regardless of the body-building material used as the binder is solely responsible for holding the three-dimensional objects together.
While improvements have been made in the process, apparatus and materials for the development of three-dimensional objects, there continues to be a need for processes and apparatus which will improve the quality of the completed objects, are easy to use, simple in design, and cheap to purchase and run. In particular, there is a need for a process, where three-dimensional objects are formed layer-by-layer. Also, there is a need for a process, which is able to use a board array of materials, including both organic and inorganic substances as well as their mixtures, as the base material. Further, there is a need for the provision of a process for forming three-dimensional objects based on electrophotographic printing, where deformations found in completed three-dimensional objects caused by spillover effects of the building process are minimized by implementing a holistic approach in process planning Additionally, there is a need for a process for forming three-dimensional objects based on electrophotographic printing, where either conductive or non-conductive materials are used as the filler material, so that the selected filler has physical and chemical properties that are compatible with the base material, and there is obtained three-dimensional objects possessing attributes that are comparable to those produced by conventional manufacturing techniques, such as die-casting or CNC machining. Also, there is a need for a process, where the base powder is dispensed in such a manner that those not bonded during the build process, remain in the work space and serves as a support structure for ensuing layers.